Vacuum furnaces are known and used for heat treating metallic workpieces at temperatures in excess of 1500.degree. F. At such temperatures the metallic workpieces are highly reactive and scale forms easily on the surfaces of the workpieces when they are exposed to air. Scale adversely alters the surface of the workpieces, and if the workpieces are to be hot formed, the scale interferes with the forming process. Therefore, in most situations the scale must be removed and is usually removed by an expensive process involving the use of hazardous chemicals.
Some alloys undergo a phase transition when cooled from the heat treating temperature to below a certain temperature, typically in the range of 300.degree. to 650.degree. F. Once the material undergoes this phase change, it is substantially less ductile and is more difficult to mechanically work such as by forging or rolling. Therefore, many such alloys must be mechanically worked above the phase transition temperature.
Two stage heat treating furnaces are well known in the art. The two stage furnace avoids the problem of scale formation because the metallic workpieces are cooled to room temperature in the furnace without exposing them to air. A typical two stage furnace has a heating chamber and an adjoining quenching chamber. The heating chamber is typically enclosed in a vacuum vessel and the quench chamber may also be enclosed in a vacuum chamber or it may be enclosed in a pressure vessel to facilitate cooling by an inert gas under positive pressure. The workpieces are heated to an elevated temperature in the heating chamber and are then transferred to the quench chamber where they are cooled. Typically, the workpieces are quenched rapidly, such as by immersion in oil.
While a two stage furnace avoids the scale formation problem associated with heat treated metallic parts, it has limited usefulness when the workpieces are to be hot or warm worked after heat treatment. The known two stage furnaces are unsatisfactory in such processes because they are designed to fully cool the heat treated workpieces. While it is known to reheat a metallic workpiece to its hot working temperature range, such a step necessarily adds considerable time and expense to the manufacturing process. Additionally, for many materials, reheating the material to the hot working temperature range will not reverse the effects of the phase change. The material must be reheated to a solution temperature, which is generally significantly higher than the hot working temperature range. To avoid the additional time and expense of reheating the workpieces to the hot forming temperature, it is preferable to maintain the workpiece at the desired working temperature in the time between completion of the heat treatment and the start of the hot forming process.
For a workpiece to be properly hot formed, the workpiece must be at an elevated temperature within its hot working temperature range. Frequently the temperature at which the workpieces must be maintained is critical and cannot fluctuate significantly. If the temperature of the workpiece falls below a critical lower temperature, the workpiece is likely to crack or otherwise be damaged during the hot working process.
The known two-stage heat treating furnaces leave something to be desired for handling workpieces that are to be hot formed because most of the two-stage furnaces include quench chambers, thereby necessitating reheating of the workpieces prior to hot working them. Additionally, the known heat treating furnaces that do not include a quench chamber have no means for maintaining the workpiece at an elevated temperature without exposing the workpiece to air once it leaves the furnace.